Dear Paul Bunyan, you must be a big boy if it says you should ride a 168...anywho, they come from the manufacturer (via the engineer/designer). It is essentially the amount of force (weight) of the rider when compared to the stiffness and function (ideally what the board was designed for) of the board. This is why park rider size down, powder riders size up, and ranges are fairly large. Every board is different in the size range as well; a stiffer 156 board could ride just as strong on edge as a softer 159 board even though it has more effective edge.
Does that help?